1. Field of the Invention
The present invention relates to a so-called sheet-through type image reading apparatus which conveys a document to be read to move on one main side of a transparent plate and reads an image of the document from the other main side of the transparent plate, and an image forming apparatus provided with the image reading apparatus.
2. Description of the Related Art
In a case where a sheet-through type image reading apparatus is used to read an image of a document, print by means of a printer, and take an image into a computer such as a personal computer to display the image on a display, a linear image along a sub-scanning direction may appear in an image. This is usually caused by adherence of foreign objects such as waste, dust, and a paper piece of a conveyed document sheet to a glass of the image reading apparatus, a scratch on the glass, and the like. The linear image may appear as a black line on a white background portion or as a white line on an image portion and is highly visually eye-catching. Therefore, for an image reading apparatus which is required to obtain image data of a document precisely, removal of such linear image is demanded.
As a technology for preventing occurrence of such linear image, there has been proposed a technology disclosed in, for example, the following patent document.
The following Japanese Patent Unexamined Publication No. 2000-196814 discloses as follows. “The sheet-through type document reader in accordance with the present invention has a means for changing a reading position by a predetermined distance only if an abnormal image is detected at a predetermined sheet-through document reading position. Thus, a black linear image due to waste (dust), dirt, or a scratch can be prevented. In other words, if waste (dust) 27 is detected at a predetermined reading position, the reading position is shifted by a predetermined distance D, and the document reading is performed. Until a new abnormal image is detected at the shifted reading position, the sheet-through type document reader continues executing the reading at the same position . . . (abbreviated) . . . . The reading (hereinafter, referred to as “pre-scanning”) is performed before the document is conveyed to the reading section. That is, document reading is performed in a state where a document is not present at the reading position, in other words, the document presenting member (white sheet 26) 23 is read directly.”
According to the technology of the patent document, a predetermined time period is required for moving the reading position by the predetermined distance D. Here, this time can be provided during a period when a document is not passing through the reading position. In other words, the time period can be provided during a period when a clearance between documents is passing through the reading position.
However, especially in image reading apparatuses available in recent years, the clearance between documents is made as small as possible to improve the number of documents (productivity) which can be read in a unit of time. In such case, it is difficult to secure the time for moving the reading position by the predetermined distance D during the aforementioned period. Therefore, for the purpose of securing the time for moving the reading position by the predetermined distance D, it is necessary to make the clearance between documents be wider or temporarily stop conveying documents each time when the clearance passes through the reading position. However, either of those may lower the productivity.
On the other hand, as a technology for preventing occurrence of a linear image other than the technology of changing the reading position, there has been a known technology of executing an image processing to remove a linear image from an image which is obtained by a reading operation without changing the reading position. If this technology is adopted, it is advantageous in that the aforementioned lowering in productivity may not occur. On the other hand, as compared to the case where the image processing for removing a linear image is not executed, there is a disadvantage that, in view of characteristics in image processing, image quality of an image is likely to be lowered.